Telecommunications systems incorporate extensive optical fiber networks using frequency multiplexing/demultiplexing techniques for optical communications signals. These types of optical communications systems require add/drop filters for selecting a single wavelength from complex optical signals that are typically frequency multiplexed together. Also, optical sensors are used at narrow band frequencies and wavelengths and may require add/drop or other functionality. These sensors are required for accelerometers, chemical and biological sensors, and similar applications.
Prior art devices for these add/drop filters and optical sensors include Fabry-Perot structures, waveguide ring resonators, and spherical resonators. Fabry-Perot structures have been widely used for many applications, but have difficult extensions to the multipole. Waveguide ring resonators are planar structures that can be fabricated with little complexity and incorporate a simple extension to the multipole. One drawback is their high losses. Spherical resonators are small in size and have low loss, making them efficient for limited applications. They are not efficient, however, for some applications requiring an extension to multipole filters. Other microcavity geometries incorporate whispering gallery modes and photonic crystals.
FIG. 1 shows a prior art microsphere 20 positioned adjacent an optical fiber 22. A 980 nm SMF and 980 nm optical pump are used as an input and the output is a 1550 nm SMF and 1550 nm laser. The optical fiber 22 is tapered and can be brought in contact with the microsphere 20 and the evanescence light from the optical fiber 22 enters the microsphere 20. Air guided region 24 and vestigial cores 26 are shown. The TEill mode of propagation occurs along the “equator” or center portion of the microsphere. This is a well known practice operative in a whispering gallery mode.
Other examples of prior art microspheres operative in a whispering gallery modes have been designed. For example, U.S. Pat. Nos. 6,389,197; 6,487,233; and 6,490,039 assigned to California Institute of Technology, disclose the use of microspheres based on whispering gallery mode microresonators or cavities. An optical probe can be evanescently coupled into at least one whispering gallery mode of the resonator. Optical energy can also be coupled in a waveguide mode, into the resonator that operates in the whispering gallery mode. For example, a fiber in its waveguide mode would couple information to the resonator, e.g., the microsphere. The fiber can be cleaved at an angle to cause total internal reflection within the fiber. The energy in the fiber forms an evanescent field and the microsphere is placed in the area of the evanescent field. If the microsphere resonance is resonant with energy in the fiber, information in the fiber is effectively transferred to the microsphere. Surface gratings can also be placed on the microsphere. This is advantageous because microsphere resonators can have high quality (“Q”) factors and small dimensions. They can be a building block for larger fiber optic systems. It is also possible to have a fiber-coupled laser based on a whispering gallery mode resonator formed of a laser gain medium and an angle-polished fiber coupler as disclosed in the '233 patent. The optical fiber can be configured to guide light at both the pump wavelength and a laser wavelength, including an angle-polished facet that forms an angle with respect to the fiber such that the angle-polished facet is positioned with respect to the other resonator to couple evanescently pump light at the pump wavelength in the optical fiber into a whispering gallery mode at the pump wavelength, and also evanescently couple light in a whispering gallery mode at the laser wavelength into the optical fiber.
One prior art improvement over the use of microspheres are toroid microcavities. These microcavities can have ultra-high “Q” factors of about 100 million and a surface-tension induced microscale cavity. Examples include droplets, silica microspheres, and microtoroids.
Toroid microcavities have been formed by photolithography and etching techniques on an oxidized silicon wafer to create silica disks. A gas XeF2 etch undercuts the silica disks with an induced reflow of the silica using Co2 to cause a smooth toroidal periphery. Toroid microcavities support whispering gallery type modes on a silicon platform and can reduce the mode spectrum compared to spherical microcavities. Microtoroids can also exhibit reduced mode volume compared to microspheres. Two mode-volume compression regimes can include slow compression and fast modal compression.
In a tapered fiber coupling, the fiber tapers in a transition from conventional core guiding regions to air-guiding regions with a vestigial core on either end as shown in FIG. 1. It can include coupling both to-and-from a microtoroid on a chip.
These ultra-high “Q” factor and small mode volume results in high circulating intensities because of the cavity build-up factor. Optical fibers that are tapered result in an ultra-low loss and optimum coupling of the microcavities. The cavity build-up factor and non-linear threshold level can be exceeded as indicated from the equation below:
            (                        P          cav                          P          in                    )        =                  λ                              π            2                    ⁢          n          ⁢                                          ⁢          R                    ·                        Q          ex                                      (                          1              +                                                Q                  ex                                                  Q                  o                                                      )                    2                                P      in        =          1      ⁢                          ⁢      mW      ⁢                          ⁢                        P          circ                ~        110            ⁢                          ⁢      W                          V        m            ~      650        ⁢                  ⁢          µm      3        ⁢                  ⁢                  I        circ            ~      2.5        ⁢                  ⁢    GW    ⁢          /        ⁢          cm      2      
There have also been some experiments on stimulated Raman scattering in microspheres. The stimulated Raman scattering causes red shift of a pump (100 nm shift in a telecommunications band). Threshold levels can be typically 100 microwatts for UHQ microtoroids and high quantum efficiencies result because of an ideal coupling junction. Similar results can occur with toroid microcavities. The stimulated Raman scattering for toroid emission is typically single mode.
A prediction of threshold using bulk Raman gain constant (doubly resonant process) can be:
      P    thresh    =                                          π            2                    ⁢                      n            2                    ⁢                      V            eff                                                λ            P                    ⁢                      λ            R                    ⁢                      fgC            ⁡                          (              Γ              )                                          ·              Q        ex        P            ·                        (                      1                          Q              t              P                                )                2            ·              1                  Q          t          R                      ⁢    α    ⁢                  ⁢                  V        eff                    Q        2            
A minimum threshold undercoupled could be:Qexmin=2Q0(T≅11%)
The Raman threshold can also affect the mode volume as follows:
      V    eff    =                    P        thresh                  m          ⁢                                          ⁢          i          ⁢                                          ⁢          n                    ·              Q        o        2              ⁢                            λ          P                ⁢                  λ          R                ⁢                  fgC          ⁡                      (            Γ            )                                                π          2                ⁢                  n          2                      ⁢          4      27      
P: Raman threshold
λp, λR: pump and Raman emission wavelength
g: Raman gain coefficient
C(Γ): intermode coupling parameter
Q: Quality factor of pump and Raman mode
Stimulated Raman threshold can be used to infer the mode volume Veff.
      V    eff    =                    P        thresh                  m          ⁢                                          ⁢          i          ⁢                                          ⁢          n                    ·              Q        o        2              ⁢                            λ          P                ⁢                  λ          R                ⁢                  fgC          ⁡                      (            Γ            )                                                π          2                ⁢                  n          2                      ⁢          4      27      
Although spherical resonators, waveguide ring resonators, Fabry-Perot structures and toroid microcavities have been advantageously used as indicated above, these devices still have limitations when optical fibers are coupled, even though these devices often are easily fabricated.
Published patent application no. US 2002/0041730, published Apr. 11, 2002, discloses a method for fabricating an optical resonator on an optical fiber by generating a differential of a physical property, for example, the diameter, density, refractive index, or chemical composition of a transverse segment of the resonator fiber. This could include some type of grooves forming the resonators. The resonator fiber segment can substantially confine a circumferential optical mode propagating around the resonator fiber segment circumference at least partially within the resonator fiber segment. This enables substantial confinement of a substantially resonant circumferential optical mode near a surface of the fiber. As a result, evanescent optical coupling can occur between circumferential optical modes and an optical mode supported by the second optical element. Different techniques for spatially, selectively generating the differential could include masking/etching, masking/deposition, laser machining, laser patterning and combinations of the different processes. It is also possible to include a plurality of resonators in the same fiber sufficiently close together to enable optical coupling between them to provide a frequency filter function for optically coupling multiple optical elements, including optical fibers. Although the optical resonator can provide some coupling, it is limited in its use and may not provide adequate coupling for input/output functions. Its manufacturing requires non-rotating upper and lower capillary tubes to hold a spinning optical fiber, which may not ensure accuracy and have excess tolerance. Some limited teaching for using a single, tapered optical fiber near the microcylinder is also proposed. It also does not address polarization issues, slower waveguide structures, multiple node contacts, and the use of coatings for imparting waveguide resonance or similar issues.